Such a method and such an apparatus have already been known (DE-A-3,739,825). In the nozzle head of this apparatus, individual nozzles are arranged at a setting angle relative to the direction of the main jet of the nozzle head so as to achieve a comparatively wide "scattering" of the bundle of discrete jets before the same "fan out" to such an extent that the marginal areas of the discrete jets will overlap.
In this connection it has also been known with other apparatus of similar species (DE-B-3,410,981 and 3,516,572) to employ cutting metal inserts for the nozzles and to anchor the same in the nozzle head by screwing-down or push-fitting.
Moreover, apparatus for drilling holes in concrete and rock have been known (MACHINE DESIGN 57/1985, pp.114-117), in which water jets mixed with abrasive particles are highly pressurized and are used for drilling by means of a rotating nozzle head, wherein a water pressure of approximately up to 100 bar is used.
Finally, it has also been known (CH-A-370,717 and GB-A-718,735) to atomize a liquid by means of air for the treatment of surfaces; here, too, rotary nozzles are employed by means of which the inner wall of the bore in a workpiece is treated to cause said bore to adopt the final fine-machining condition. Hence, a proper material-removing cutting effect is not obtained thereby.